Water-based polymer thick film conductive ink

ABSTRACT

The present invention relates to an aqueous conductive polymer thick film-forming composition comprising a water-soluble thermoplastic polymer, a polymer dispersion in water, a glycol drying-retarder agent, an electrically conductive amount of conductive metal and/or carbon particles and water. The composition in the production of a highly printable conductive thick film ink.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to aqueous polymer thick filmconductive compositions containing conductive metal, carbon or graphiteparticles. The compositions are particularly useful for screening intoelectrical circuits and for formulation into human contact electrodes.

2. Brief Description of the Art

In the past, water-based conductive polymer thick film formulations havetypically exhibited poor rheology, poor adhesion and poor flexibilitywhen printed and cured on polyester or polycarbonate films. They alsohave not offered the relatively low electrical resistance that isachievable with solvent based formulas.

In view of the above, new aqueous compositions which provide a solutionto the above problems while containing a reduced amount of organicsolvents, relative to conventional solvent-based thick filmformulations, would be highly desired by the polymer thick filmmanufacturing community. The present invention provides one solution tothis problem. This invention relates to the use of water solublethermoplastic polymers in a water-based polymer thick film-formingcompositions to impart smooth, easily spreadable coating qualities tothe formulation.

SUMMARY OF THE INVENTION

In one aspect, the present invention relates to an aqueous conductivepolymer thick film-forming composition comprising (preferably consistingessentially of):

(a) a water soluble thermoplastic polymer in an amount of between about0.25% and about 20% by weight

(b) a polymer dispersion in water in an amount of between about 10% andabout 70% by weight;

(c) a glycol drying-retarder agent(s) in an amount of between about 2%and about 20% by weight;

(d) an electrically conductive amount of conductive particles selectedfrom the group consisting of conductive metal (preferably silver flake),carbon, and graphite particles, and combinations thereof, with theproviso that the amount of said metal particles does not exceed 90 % byweight and the amount of said carbon or graphite particles does notexceed 40% by weight; and

(e) water in a solvent effective amount,

each of said amounts being based upon the total weight of saidcomposition.

This and other aspects will become apparent upon reading the followingdetailed description of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the present invention, an aqueous conductive polymerthick film-forming film composition is provided which provides excellentfilm properties, such as resistivity and printability, during use. Thecompositions provide a distinct advantage from an environmentalstandpoint since they are water-based. These compositions are useful ashighly printable conductive inks utilizing screen printing or otherconventional printing method. These inks are of such highly printablequality that lines and spaces 15 mils wide can be clearly printed. Thishighly printable quality is important in order to effectively print theintricate circuitry patterns often demanded by the electronics industryin high performance applications such as, for example, in thepreparation of touch pad circuitry.

Examples of suitable thermoplastic water soluble polymers useful in thepresent invention include polyethyloxyazoline, polyvinyl pyrrolidone,polyvinyl alcohol, polyacrylamide, polyglycols and polyacrylic acid.

Examples of suitable polymer dispersions in water include polyurethane,acrylic, polyester and vinyl resin polymers.

The purpose of the glycol drying retarding agent(s) employed in thepresent invention is to slow down the drying process so as to permitmanipulation of the polymer thick film, such as by silk screening,before it has a chance to dry. Examples of suitable glycol type dryingretarding agents include: diethylene glycol ethyl ether acetate;diethylene glycol butyl ether acetate; ethylene glycol butyl etheracetate; diethylene glycol monoethyl ether; ethylene glycol monoethylether; dipropylene glycol methyl ether; tripropylene glycol methylether; dibutyl phthalate; diocyl phthalate; diocyl phthalate; tributylphosphate; 1,3 butylene glycol; propylene glycol and ethylene glycol.These agents may be used singularly or in combination with each other.

Any conductive metal particle may be used in the composition of thisinvention such as copper, gold, nickel, silver alloys, silver platedmetals and silver. Silver is the preferred conductive particle for mostuses. Alternatively, carbon black or graphite may also be used, alone orin combination with metal particles.

The average particle size of the metal or carbon or graphite particlecan vary over a wide range but, is preferably in the range of from about1 micron to about 50 microns, more preferably from about 2 microns toabout 20 microns. If the average particle size of the metal particles isless than about 1 micron, the electrical properties may be adverselyaffected. In other words, if the metal particles are too fine, theresistivity of the composition may be too high. If the average particlesize of the metal particles is above about 50 microns, then thecomposition will be too difficult to apply to the substrate. Forexample, the composition will tend to clog up during a silk screeningprocess or the like.

Other minor conventional components may be added to the conductivecomposition. These include viscosity modifying agents and defoamingsurfactants. The viscosity of the final conductive composition of thisinvention for screen printing is preferable from about 8,000 to about25,000 centipoises at 30° C. (Brookfield RVT Spindle #6 at 20 rpm) inorder to be screen printable.

The compositions of this invention may be applied to substrates by avariety of techniques, such as silk screening, spraying or brushing.Once the conductive polymer thick film has been applied to thesubstrates it is cured at between about 93° C. and 135° C. for a timesufficient to complete curing with good adhesion.

The conductive compositions of the present invention may be applied toconventional rigid or flexible substrates. Whether the substrate isflexible or rigid in nature, the substrate may or may not have to bepretreated or precoated with any other substance before applying thepresent composition.

EXAMPLE 1 Preparation of an Aqueous Polymer Thick Film-formingComposition Using Silver Flake

A 40% solution of Peox in water was made using a cowles mixer. Thissolution was then vigorously hand mixed with the rest of the mixture'scomponents as identified in the table hereinbelow. The resulting mixturewas then passed through a three roll mill to provide the desiredfilm-forming composition.

    ______________________________________                                        Composition           Weight                                                  Component             in Parts Per 100/Wt                                     ______________________________________                                        Peox (Polyethyloxyazoline) (1)                                                                      2.80                                                    AW875 (2)             20.38                                                   Silver Flake (3)      55.25                                                   H.sub.2 O             16.22                                                   Diethylene Glycol Mono Ethyl Ether                                                                  5.22                                                    BYK020 (4)            0.13                                                    ______________________________________                                    

The properties of the film-forming composition were as follows:

    ______________________________________                                        Electrical resistivity                                                                         19 milliohms/sq. @ 1.0 mil                                   Printability     Very good                                                    Adhesion         <2 in resistance                                             Print Quality    Good                                                         ______________________________________                                    

EXAMPLE 2 Preparation of Another Aqueous Polymer Thick Film-formingComposition Using Silver Flake

A 55% solution by weight of PVP K-30 in H₂ O was made using a cowlesmixer. This solution was then agitated with the rest of the mixture'scomponents as identified in the table hereinbelow. The resulting mixturewas then passed twice through a three roll mill to provide the desiredfilm-forming composition.

    ______________________________________                                        Component       Parts per 100                                                 ______________________________________                                        PVP (5)         1.09                                                          AW875           28.15                                                         Ethylene Glycol 7.52                                                          H.sub.2 O       1.96                                                          Witco 748 (6)   .6                                                            Triton CF-10    .6                                                            Silver Flake    60.08                                                         ______________________________________                                    

The properties of the film-forming composition were as follows:

    ______________________________________                                        Viscosity       10,150 CPS                                                    (Brookfield # 6 spindle @ 20 rpm, 25° C.)                              Resistivity     17 milliohms/sq. @ 1.0 mil                                    Adhesion        <2 in resistance                                              Flexibility                                                                   1 crease        +10%                                                          3 crease        <+150%                                                        ______________________________________                                    

EXAMPLE 3 Preparation of an Aqueous Polymer Thick Film-formingComposition Using Carbon Particles

A 55% solution of PVP K-30 in H₂ O was prepared using a cowles mixer.This solution was then agitated with the rest of the mixture'scomponents as identified in the table hereinbelow. The resulting mixturewas then passed twice through a three roll mill to provide the desiredfilm-forming composition.

    ______________________________________                                        Component        Parts per 100                                                ______________________________________                                        Carbon (XC-72) (8)                                                                             6.26                                                         AW875            60.88                                                        Triton CF-10     2.60                                                         PVP K-30         2.46                                                         Ethylene Glycol  16.31                                                        H.sub.2 O        10.34                                                        Witco 748        1.15                                                         ______________________________________                                    

The properties of the film-forming composition were as follows:

    ______________________________________                                        Resistivity      1700 ohms/sq. @ 1.0 mil                                      Printability     Good                                                         ______________________________________                                    

The following is a list of sources for the various trademarks/tradenamesused in the above three examples.

1) "Peox" is a water and solvent soluble polyethyloxyazoline previouslyavailable from Union Carbide, 39 Old Ridgebury Road, Danbury, Conn.06817-0001.

2) "AW875" is a vinyl resin dispersed in water, available from UnionCarbide, 39 Old Ridgebury Road, Danbury, Conn. 06817-0001.

3) "Silver flake" is available from Chemet Corporation, 52 GardnerStreet, Attleboro, Mass. 02703.

4) "BYK02011 is a defoamer available from BYK Chemie USA, 524 SouthCherry Street, Wallingford, Conn. 06492.

5) "PVP K-30" is a polyvinyl pyrrolidone polymer, soluble in H₂ O,available from GAF Chemicals Corporation, 1361 Alps Road, Wayne, N.J.07470.

6) Witco "Bubble Breaker 748" is a defoamer available from WitcoOrganics Division, 3230 Brookfield Street, Houston, Tex. 77045.

7) "Triton CF-10" is a surfactant available from Union Carbide, 39 OldRidgebury Road, Danbury, Conn. 06817-0001.

8) "XC-72" is a carbon powder available from Cabot Corporation, 125 HighStreet, Boston, Mass. 02110.

In addition, as used in the above examples, the "adhesion" test refersto the percentage change in resistance after applying Scotch tape to atest pattern of the ink and then removing the tape and remeasuring theresistivity. Similarly, the "flexibility" test measures the percentagechange after creasing the test pattern which was silk screened for eachexample.

While the invention has been described above with reference to specificembodiments thereof, it is apparent that many changes, modifications andvariations can be made without departing from the inventive conceptdisclosed herein. Accordingly, it is intended to embrace all suchchanges, modifications and variations that fall within the spirit andbroad scope of the appended claims. All patent applications, patents andother publications cited herein are incorporated by reference in theirentirety. All parts and percentages are by weight unless otherwiseexplicitly stated.

What is claimed is:
 1. An aqueous conductive polymer thick film-formingand printable composition comprising:(a) about 0.25% to about 20% byweight of water-soluble thermoplastic polymer selected from the groupconsisting of polyethyloxyazoline, polyvinyl pyrrolidone, polyvinylalcohol, polyacrylamide, polyglycol, and polyacrylic acid; (b) about 10to 70% by weight of a water insoluble polymer dispersion in water, saiddispersed polymer selected from the group consisting of polyurethaneresin polymer, acrylic resin polymer, polyester resin polymer, and vinylresin polymer; (c) about 2% to about 20% by weight of at least oneglycol compound drying-retarder agent; (d) an effective electricallyconducting amount of less than 90% by weight of conductive particlesselected from the group consisting of conductive metal particles, carbonblack particles, graphite particles, and combinations thereof; with theproviso that the amount of said carbon black particles, graphiteparticles, or combinations thereof do not exceed 40% by weight; and (e)an effective solvating amount of water to dissolve water-solublethermoplastic polymer (a); each of said amounts being based upon thetotal weight of said composition.
 2. The composition of claim 1 whereinsaid water-soluble thermoplastic polymers is polyethyloxyazoline.
 3. Thecomposition of claim 1 wherein said water-soluble thermoplastic polymersis polyvinyl pyrrolidone.
 4. The composition of claim 1 wherein saiddispersed polymer is vinyl resin polymer.
 5. The composition of claim 1wherein said glycol drying-retarding agent is ethylene glycol.
 6. Thecomposition of claim 1 wherein said conductive particles are silverflake.
 7. The composition of claim 1 additionally comprising viscositymodifying agents and defoaming surfactants.
 8. The composition of claim1 wherein said conductive particles are from about 1 micron to about 50microns.